When you just enable the Approvals column and don't use a workflow, the people who should approve must go into the library to see and approve or reject the new or changed item. A workflow can make the process easier, and the quickest way to add an approvals workflow to a document library is to use the built-in one.

OOTB workflow creates review task
SharePoint only has a few workflows that can be used out of the box, but there is an Approvals workflow to use in document libraries where the Approvals column is enabled. This workflow creates a task for the approver, and from that task the approver can open the file in question and approve or reject it. The approving person can also select to suggest changes and to send the file to another reviewer.

The built-in workflow can create review tasks for several people and at multiple stages. There are several more settings, like due date and what will happen if the file is rejected by someone or is changed when the workflow is still running.

Drawbacks
All this sounds just fine, but there are some drawbacks. It is not possible to specify workflow conditions, for example that the workflow will only run for a specific file type, and users who can start the workflow manually can also change the workflow settings. Another problem is that you cannot see who has approved the file without opening the workflow page.